The present invention relates generally to an apparatus for controlling a braking hydraulic pressure to be applied to each of wheels of a motor vehicle.
Anti-skid control systems are known as systems for principally controlling the braking hydraulic pressure in a brake control system to prevent the motor vehicle from being slipped laterally due to locking of a vehicle wheel on a rapid braking operation. For effectively performing the braking operation, it is desirable that the braking hydraulic pressure is increased in advance to a pressure value immediately before occurrance of the locking of the vehicle wheel and is kept for a long time as it is. However, since the locking pressure is varied in accordance with the running road surface state, initially taking a long time to increase the hydraulic pressure may result in excessive increase of the hydraulic pressure and locking of the vehicle wheel. Therefore, it is not allowed to increase the hydraulic pressure at once and it may be required to gradually increase the hydraulic pressure in accordance with the slipping state of the vehicle wheel. This does not result in effective braking control.
An attempt to avoid this problem is disclosed in Japanese Patent Provisional Publication No. 60-47751, where, as shown in FIG. 13 of this application, the locking prevention apparatus is arranged such that in the present pressure increasing control cycle, a time ta taken for increasing the pressure is determined as ta=f(n) where f is a constant function and n (=4) is the number of times that the hydraulic pressure has been increased in the previous pressure increasing control cycle. However, in such a locking prevention apparatus there arises problems because of no consideration of the previously decreased hydraulic pressure P1 and the present decreased hydraulic pressure P2. More specifically, if there is the difference in the braking responsibility between two types of the motor vehicles, the difference results in variation of the present decremented hydraulic pressure (P2, P3). That is, the decrement of hydraulic pressure is varied, for example, between the first type in which the releasing of the braking force is highly responsive to the decrease of the hydraulic pressure and the slipping of the vehicle wheel is settled rapidly and the second type in which the responsibility is slightly inferior to the first type and the wheel slipping is settled gradually. Furthermore, the inertia force of the driven wheel is varied in accordance with the gear position of the transmission and the settlement of the wheel slipping is made under different conditions. For example, the slipping is rapidly settled under the condition of the neutral gear position and the slipping is rgradually settled due to the engine braking action under the condition of the low-speed gear position. Therefore, when applied to a motor vehicle, the conventional apparatus requires to appropriately and accurately obtain the function f(n) in consideration of braking responsibility and so on. If the function f(n) is not appropriate, there provides a problem in that, when as indicated by P2 in FIG. 13 the presently decreased amount of the hydraulic pressure is small, the next pressure increasing time period ta results in the fact that the braking hydraulic presure exceeds the lock limit pressure, and when as indicated by P2 in the figure the present decrement is great, the pressure increasing time period ta results in the fact that the braking hydraulic pressure is separated insufficiently from the lock limit pressure.